1. Field of the Invention
The present invention relates to the field of hearing aids. The invention, more specifically, relates to hearing aids utilizing noise reduction techniques. The invention further relates to methods for adjusting the hearing aid gain for noise reduction. In addition the invention relates to a system of reducing noise in a hearing aid.
Hearing aids are adapted for providing at the users eardrum a version of the acoustic environment that has been amplified according to the users prescription. This is normally achieved by providing a device with a microphone, an amplifier and a miniature loudspeaker situated in an earpiece placed in the users ear canal. It is well known that there may be acoustic leaks around the earpiece. There may e.g. be a non-sealed fit or there may, for considerations about user comfort, be a vent deliberately arranged in the ear piece for relieving the sound pressure created by the users own voice. Such leaks may cause a loss in sound pressure and they may allow sound to bypass the hearing aid to reach the ear drum.
2. Description of the Related Art
PCT application PCT/EP2005/055305, published as WO-A1-2007/045271, titled “Method and system for fitting a hearing aid”, the contents of which are incorporated herein by reference, provides a method for estimating otherwise unknown functions such as the vent effect and the direct transmission gain for an in-situ hearing aid. The vent effect estimate is used for correcting the in-situ audiogram and the hearing aid gain.
WO-A1-2005/051039 provides a dynamic speech enhancement technique, where speech intelligibility in noise is improved by optimizing a speech intelligibility index; such as SII (see also Methods for Calculation of the Speech Intelligibility Index: ANSI S3.5-1997), AI (see also American National Standard Methods for the Calculation of the Articulation Index; ANSI S3.5-1996). Noise reduction techniques, where speech intelligibility in noise is improved by optimizing a speech intelligibility index, increase or decrease the gain in selected frequency bands, taking into account human auditory masking.
The sound input to the hearing aid user is a combination of the sound amplified according to the hearing aid gain together with the direct transmitted sound. As long as the amplified sound dominates the direct transmitted sound in all frequency bands, the noise reduction techniques will provide good results. Noise reduction according to the state of the art to enhance SII is based on an assumption that the earplug provides a tight fit between the earplug and the ear canal. However a ventilation canal or a leakage path allows for the sound to be directly transmitted into the ear. Thus, at a certain threshold the sound input to the hearing aid user may be dominated by the direct transmitted sound, so that a decrease of the hearing aid gain will not affect the sound input to the user. If the direct transmitted sound is not taken into account, the speech intelligibility may suffer as a consequence.
Therefore, acoustic effects of the ventilation canal and possible leakage paths between the hearing aid and the ear canal are still challenges in today's hearing aid fitting strategies.
Thus, there is a need for improved hearing aids as well as improved techniques for implementing noise reduction in hearing aids.